1. Field of Art
This description generally relates to energy storage, and particularly to energy storage using flywheels.
2. Description of the Related Art
Many energy sources, particularly clean energy sources such as wind turbines and solar panels, generate energy that does not temporally match the load experienced. In much of the developed world, energy generation follows experienced load, such that energy is provided as needed. Under circumstances of high load, techniques such as the use of peaker generators and spinning and non-spinning reserves on thermal generators allow for generation that matches high and variable load. However, despite the availability of such techniques, there are often instances where energy storage is important for meeting energy load.
Existing energy storage systems all have drawbacks of one form or another. Size, price, storage efficiency, efficacy, and safety are all concerns when designing an energy storage system. Generally, smaller size, lower price, reduced loss in both inputting energy for storage and extracting it for distribution, reduced losses for continuous operation, and safe disposal are all preferred characteristics of energy storage systems.
A flywheel is one type of energy storage system that stores energy as rotational kinetic energy. A flywheel rotor is a weighted, rotationally symmetric mass that spins while physically coupled, directly or indirectly, to a motor/alternator that itself is electrically coupled to a converter, such as a back-to-back inverter system, constituting an AC-AC conversion subsystem. When power is received for storage, the rotor is driven, increasing the rotational speed of the flywheel rotor. When power is to be extracted, the flywheel rotor drives the motor/alternator. The faster a flywheel rotor can spin, the more energy it can store, but the more stress is induced on the rotor. Generally, the amount of stress a rotor is able to sustain while operating is a function of the design, materials, and processes used to make the rotor. Specifically, the amount of stress that can be sustained depends on a combination of the rotor material's yield strength, fracture toughness, maximal intrinsic defect size, cyclic fatigue characteristics, and the rotor's shape, among other factors.
Generally, rotor thickness is constrained by the choice of materials from which it is manufactured. For example, rotor thickness when using low-alloy steel materials such as 300M steel is generally limited to 8-14 inches due to the constraint of through-hardening during the quench step. If using an alloy such as AISI 4340, thickness is further limited to about 4-6 inches.
In most applications, multiple flywheel units are required to meet the overall energy storage requirements. The overall system cost in such applications can be reduced by building larger flywheels units, i.e. individual units that store more kinetic energy by incorporating greater rotational, or rotor, mass. Since the thickness of a single rotor is constrained, other approaches may be used to increase flywheel rotor mass, and consequently increase the energy storage capacity of an individual flywheel unit. One way is to scale up diameter, while keeping thickness constrained. Scaling to large diameters is effective with respect to engineering design and generally known manufacturing techniques, but requires substantial capital investment in tooling to perform steps like forging and machining to final net shape. Shapes with large lateral dimensions can present logistical challenges in shipping.